The present invention relates to an ASK (Amplitude Shift Keying) modulation apparatus, particularly to an ASK modulation apparatus and method for a mobile terminal used for a microwave band with reduced unintended radiation and improved on/off ratio.
Recently, in the data communication system which uses a microwave band radio signal for duplex high speed data communication between a master station and a plurality of small mobile terminal, particularly, because of small size and low power consumption, a binary ASK modulation system is used as a modulation system in the mobile terminal.
In such a data communication system, data is communicated between the master station and mobile terminals by using two or more ASK modulated channels in the non-operating section with a certain frequency interval.
If a rectangular pulse as shown in FIG. 18A is used as a modulation pulse, in a general ASK modulator, the frequency spectrum of a transmission signal as shown in FIG. 18B is that a rectangular pulse (modulating wave) is developed as Fourier series.
But, this frequency spectrum includes an odd high level higher harmonic component in addition to a modulation component that is necessary for data transfer. This higher harmonic component raises a problem of adjacent-channel interference with other communication channels or failure to satisfy the occupied frequency bandwidth defined by the radio law.
On the other hand, the binary ASK modulated wave is obtained by turning on/off a carrier wave, but if a carrier leak is large while the carrier is turned off, the carrier detection circuit of the demodulator in the receiver side of the master station may malfunction, and the carrier leak must be sufficiently lowered.
A method has been proposed to reduce the unintended out-band radiation due to the higher harmonic component and carrier leak power.
For example, to reduce the higher harmonic component, the JP-A-No.167645/1992 discloses the method to apply a modulation pulse of rectangular wave to an ASK modulator through an integration circuit composed of a resistor and a capacitor.
FIG. 19 is a block diagram showing the ASK modulating circuit mentioned in the above reference.
A data signal is applied to the base of a transistor 403 through a base resistor 407 via an integration circuit 404 composed of a resistor 405 and a capacitor 406. The collector of the transistor 403 is connected to a transmission line 402 that input the output of a carrier signal generator 401.
When the data signal is applied through the integration circuit 404, a smooth modulated pulse is generated to reduce the unnecessary higher harmonic component level.
The method of the above reference decreases the higher harmonic component to a certain extent, but a desired characteristic can not be obtained in the data communication system that the present invention is applied. The reasons are described as below.
FIG. 20 shows the primary required characteristics of the ASK modulation apparatus adopted in the data communication system of the present invention. In FIG. 20, a carrier frequency is 6 GHz and a data modulation rate is 2 Mbps. The present modulation appratus performs binary ASK modulation under this condition. As the adjacent channel is allocated at 10 MHz away from the center frequency (6 GHz), the D/U ratio of the maximum power (non-modulated carrier power) to the adjacent channel leak power (hereinafter, referred to a D/U ratio) needs to be 40 dB or higher depending on the condition not to deteriorate the code error rate when transmitting data through the adjacent channel.
The on/off ratio of the maximum power when the carrier is off (non-modulated carrier power) to the carrier leak power (hereinafter, simply referred to an ON/OFF ratio) needs to be 20 dB or higher.
Further, a 99% occupied frequency bandwidth (hereinafter called an occupied bandwidth) must satisfy severe 8 MHzp-p or lower that is four times of 2 Mbps.
When using the ASK modulating circuit having the primary integration circuit described in the above patent, it is theoretically impossible to obtain the D/U ratio of 40 dB or higher while satisfying the occupied-bandwidth. Besides, it is also impossible to satisfy the required on/off ratio, because a high frequency component leaks to the output through the transmission line even while the carrier is off. This is the same even if the integration circuit is replaced by a secondary simple low-path filter.
With the primary or secondary simple integration circuit, for a step-like input signal such as a digital signal, the response waveform on the time axis for output signal rapidly changes, fails to smooth the signal completely, attenuates the high frequency component of a base band signal, and provides no effect to limit the frequency band. Further, as the input impedance of the band limit filter is not constant over a wide range of frequency, it is apt to cause ringing, overshoot and waveform disturbance against a base band signal like a digital signal including a high frequency component. This will unnecessarily expand the occupied frequency band. Thus, the desired characteristic can not be obtained.
As a method of decreasing the unnecessary out-band radiation due to the higher harmonic component, a digital filter is used because it is excellent in the band limit characteristic and is relatively easy to change data.
With the ASK modulation apparatus using a digital filter, the characteristic of the out-band higher harmonic component is satisfied, but a D/A converter and a memory to store data to meet the required spurious characteristic become necessary, the compactness as a mobile terminal will not be realized, and the battery driven low power consumption requirement will not be achieved.
An object of the present invention is to provide a microwave band ASK modulation apparatus and method which satisfies compactness, low power consumption and required modulation characteristics such as D/U ratio, occupied bandwidth and on/off ratio by using a passive element and not by using a large complex circuit such as a digital filter.
The objective of the present invention is achieved by an ASK modulation apparatus which modulates a rectangular high speed data signal within a microwave band comprising:
a band limiting means for waveform shaping to eliminate the inclination of an output waveform at rising of data signal by using a passive element, and
an ASK modulating means for inputting carrier wave of a microwave band and ASK-modulating said carrier wave by turning off the drive voltage of a high frequency amplifier based on the output of said band limiting means.
The ASK modulation apparatus of the present invention uses a band limiting means to raise a rectangular data signal waveform with zero inclination. This decrease the higher harmonic component included in the rectangular pulse of the data signal. The high frequency amplifier of the ASK modulation means is operated on the band limited output from the band limiting means, and when the rectangular wave is at a logical level xe2x80x9c1xe2x80x9d, it becomes in the operating region and amplifies the microwave band carrier. If the rectangular wave is at a logical level xe2x80x9c0xe2x80x9d, it becomes in the cut-off state and cuts off the carrier output. As a result, it provides a preferable ASK modulated wave.
The ASK modulation apparatus further includes a drive means comprising a buffer amplifier for driving said high frequency amplifier in the output of said band limiting means.
With the drive means, the bank limiting circuit is stably operated.
The band limiting means comprises the N sets (N is an integer greater than 2) of basic circuit which are connected in cascade to data signal input and the grounded resistor R being connected to the output of the Nth stage of the said basic circuit. Each of the basic circuit comprises a coil L provided between input and output and a serial circuit being one end connected to the input of said coil L and other end connected to the ground, and the serial circuit having a resistor R and a capacitor C.
With two or more stages of the basic circuit, the output signal waveform can be smoothly raised with zero inclination though the input signal changes discontinuously (step-like).
The basic circuit is composed to satisfy L=CR2.
The connection number N and the constant of said basic circuit depends on the time response of said data signal.
Further, the connection number N and the constant of the basic circuit depends on the rise time of the rectangular wave of said data signal, the suppression of the out-band unnecessary higher harmonic component and adjacent carrier component of said ASK modulation wave.
The buffer amplifier comprises is two transistors combined in the emitter-follower type.
The buffer amplifier comprises an operational amplifier.
The buffer amplifier comprises an operational amplifier and a current amplifier.
The high frequency amplifier comprises a field effect transistor or bipolar transistor amplifier connected to M stages (M is an integer greater than 1).
To achieve the above object, the present invention provides an ASK modulation apparatus which modulates a rectangular high speed data signal within a microwave band comprising:
a band limiting circuit having four basic circuits that are connected in cascade to said data signal input, each of said basic circuits having a coil L provided between input and output and a serial circuit being one end connected to the input of said coil L and other end connected to the ground, said serial circuit having a resistor R and a capacitor C; and the grounded resistor R is connected to the output of the 4th stage of the said basic circuit, and
an ASK modulating means for inputting the carrier wave of a microwave band and ASK modulating said carrier wave by turning off the drive voltage of a high frequency amplifier based on the output of said band limiting means.
The ASK modulation apparatus further includes a drive means comprising a buffer amplifier for driving said high frequency amplifier in the output of said band limiting means.
To achieve the above object, the present invention provides an ASK modulation apparatus which modulates a rectangular high speed data signal within a microwave band comprising:
a band limiting circuit having four basic circuits that are connected in cascade to said data signal input, each of said basic circuits having a coil L provided between input and output and a serial circuit being one end connected to the input of said coil L and other end connected to the ground, said serial circuit having a resistor R and a capacitor C; and the grounded resistor R is connected to the output of the 4th stage of the said basic circuit,
a buffer amplifier having the first and second transistors connected to said band limiting circuit and combined in the emitter-follower type and
a high frequency amplifier circuit having a source grounded type field effect transistor (FET) which inputs the output of said buffer amplifier to the drain and the carrier of the microwave band to the gate.
To achieve the above object, the present invention provides an ASK modulation apparatus which modulates a rectangular high speed data signal within a microwave band comprising:
a band limiting circuit having four basic circuits that are connected in cascade to said data signal input, each of said basic circuits having a coil L provided between input and output and a serial circuit being one end connected to the input of said coil L and other end connected to the ground, said serial circuit having a resistor R and a capacitor C; and the grounded resistor R is connected to the output of the 4th stage of the said basic circuit,
a buffer amplifier having an operational amplifier connected to said band limiting means and a current amplifier connected to the output of the operational amplifier, and
a high frequency amplifier circuit having a source grounded type field effect transistor (FET) which inputs the output of said buffer amplifier to the drain and the carrier of the microwave band to the gate.
To achieve the above object, the present invention provides an ASK modulation method which modulates a rectangular high speed data signal within a microwave band comprising:
a step of band limiting for waveform shaping to eliminate the inclination of an output waveform at rising of said data signal by using a passive element,
a step of inputting the carrier wave of a microwave band, and
a step of ASK modulating said carrier wave by turning off the drive voltage of said high frequency amplifier based on said band limited signal.
The ASK modulation method of the present invention limits the band for waveform shaping with zero inclination of the output waveform in rising of data signal, thereby decreasing the higher harmonic component included in the rectangular pulse of the data signal. As a result, the occupied bandwidth of the output modulation wave and the adjacent channel leak power will be decreased.
The band limiting is characterized by; N sets (N is an integer greater than 2) of basic circuit which are connected to the said data signal input, each of said basic circuits having a coil L provided between input and output and a serial circuit being one end connected to the input of said coil L and other end connected to the ground, said serial circuit having a resistor R and a capacitor C, and the grounded resistor R is connected to the output of the Nth stage of said basic circuit.
The number of stages and the circuit constant of said basic circuit is decide by:
a step of calculating the output response assuming the time constant T as a parameter when N=2 and L/R=T,
a step of first determining whether said output response satisfies the given D/U ratio and occupied bandwidth,
a step of second determining whether said output response satisfies rises within the minimum pulse wide when the rectangular pulse is an input,
a step of deciding the number N of stages and the circuit constant of that time when said first and second determining results are both satisfied, and
a step of increasing N and repeating said first determining step and second determining step when one of said first and second determining results is not satisfied.
In said first determining, only one of the D/U ratio and occupied bandwidth can be judged on some conditions of the transmission system.